Even if you aren't concerned about the possibility of "backdoored hardware" (and I am not, for one), I'm still sympathetic to the argument that giving money to Google is morally wrong because it provides financial support for their data-harvesting business model (though since you use GrapheneOS, they'll only be able to use it to harvest other users' data, of course, not yours). Buying a used Pixel doesn't support Google financially, so perhaps that's more ethical.

See the response at https://news.ycombinator.com/item?id=44686811.